Can the clone brush sample surfaces facing away from the camera?
If you want to clone from a surface directly behind the brush, then no, this isn't supported.

However, if its slightly offset - you can do something like this using hidden faces.


disable back-face culling in the project paint options panel.
enable face-masking in the header (MKey)
select faces on the clone source, then Hide with HKey.
select faces to paint onto(note that border select works well in both cases).
paint...




Another option would be to duplicate the geometry in editmode and move it so it can be used as a clone-source, though this is isn't very convenient.